During a surgical procedure and within a certain time period thereafter, a patient's body requires to be externally heated. Heat transfer devices for transferring heat to the patient's body, which are used at present in operating rooms, are generally of two types: the first type utilizes a closed circuit with heated water circulating thereinside, and the second utilizes an open circuit, wherein heated air blows around the patient's body.
The heating technique of the first kind involves placing a thin rubber mattress underneath the patient. Warm water circulates through this mattress. A device, equipped with a heat exchanger and pump, warms and circulates the water. Some of these devices also have the capability of regulating the temperature of the water based on body temperature measured with a rectal probe. The heat exchange characteristics of the heating mattress are unsatisfactory since only the patient's back is warmed. It is used mainly in situations where it is impossible to place anything on the front surface of the patient's body. It is not as effective as a convection warm air system in maintaining body temperature intraoperatively.
As for the technique of the second type, it is carried out by a system consisting of a modified “hair dryer” that supplies warm air to specifically designed blankets. The blankets facilitate the distribution of the warm air over the non-operative portion of the patient's body. This system is capable of providing excellent heat transfer to a patient, and is used both to prevent the development of hypothermia and to warm hypothermic patients. The device can provide warm air at 37-42° C. The footprint of the device is small, and there is a pole mounted version. This device does not make much noise and is extremely easy to use.
Intraoperative warming is needed during most operative procedures lasting longer than 30 minutes, and in all operations on children weighing less than 15 kilograms. The only situation where warming is practically not needed, are short procedures (less than 30 minutes), and when the temperature of the patient's body is intentionally lowered. Most patients begin losing body heat either prior to anesthesia or immediately upon the induction of anesthesia. Much body heat is lost within an hour of the start of anesthesia. Therefore, warming must begin either before or immediately following the induction of anesthesia.
One of the problems of intraoperative warming is associated with the requirements for servo-controlled delivery of heat, aimed at preventing overheating and allowing for the increased delivery of warmth to patients who are continuously losing body heat. Another problem is associated with the need for the circumferential warming of limbs, warming of discontinuous areas of the body and the possibility of warming sterile areas of the operative field. It is desirable to place a heating device on individual parts of the patient's body, not underneath, so as to prevent the further loss of heat. Additionally, it is desirable to prevent the fan distribution of the warm air, and, while warming the desired parts of the patient's body to prevent the warming of surgeons, nurses and anesthesiologists presented at the time.
Notwithstanding the fact that electrical heating has evident advantages such as the possibility of the continuous control of heating, including computer control and automatic management electrical heating devices have never been used for intraoperative warming to maintain the required temperature of a patients body. This is due to the fact that the use of electrical heating in operating rooms needs to guarantee the following three complex tasks:
1. It should guarantee electrical safety, namely, to completely stave off the damage of the electrical voltage, especially taking into account that the patient is undressed, various solutions are in use during an operation, and the presence of various metal instruments, as well as hemorrhage.
2. It should guarantee to avert the danger of burns that could be caused by the local overheating of heating elements.
3. It should guarantee the absence of the influence of electromagnetic effects on electronic equipment involved in the operation or located in the vicinity of the heating device.
It is important to understand that in most cases, the above three tasks have to be accomplished together and completely. For example, to avoid the damage of electrical voltage, voltages as low as possible must be used. However, the use of low voltages with the required power compels the operation to take place with a high current, while this increase in current affects the electronics located in the vicinity of the heating device.
To decrease to zero the influence of the electromagnetic field of an electric power source onto the functioning electronics, it is known to utilize a bifilar arrangement of conductors. This technique is disclosed for example, in the patent document DE29610436. However, this technique unavoidably leads to local heating, owing to the fact that the conductors in which heat required for heating an object (e.g., a patient's body) is dissipated, are located very close to each other.
The use of bifilars is generally known as directed towards decreasing the level of the electromagnetic field. Bifilars therefore are widely used in domestic interior electric wiring, in field-effect electrical engineering, as well as in machines for contact welding where the current reaches the values of 10,000 A.
As defined by accepted standards with respect to this specific application of maintaining the temperature of a patient's body, to completely avoid the damage of burns to a patient's body, the temperature on a heating element itself, mainly on its external surface, should not exceed 41° C. At the same time, the surface temperature of the patient's body should be maintained at a level of 37° C. Accordingly, the heat flow from the heating element to the patient's body should be carried out with the maximum temperature gradient of 4° C. Taking into account that several layers of materials with very low heat conductivity (air, tissue, polyethylene) have to be accommodated between the heat dissipating element and the body, the exclusive complication of this problem is evident.